1. Field of the Invention
The present invention relates generally to the field of high oxygen barrier polymers. More particularly, it concerns a modified poly(ethylene vinyl alcohol) (EVOH), or blends comprising the modified EVOH, used as an oxygen barrier for food and beverage packaging applications, methods of making the modified poly(ethylene vinyl alcohol), packaging articles comprising the modified poly(ethylene vinyl alcohol), and methods of making the packaging articles.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen transmission from environment into the oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition, high oxygen barrier packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking.
Plastics continue to expand into food packaging applications traditionally served by metal and glass materials. An important packaging application area for polymeric materials is in packaging oxygen-sensitive food and beverage products. Polymers used for these applications, either as films or rigid containers, can be classified by their relative permeation to oxygen. Of the many classes of polymers. for such applications, those generally held to be high oxygen barrier materials include poly(ethylene vinyl alcohol) (EVOH), poly(vinylidene chloride) (PVDC), and acrylonitrile polymer (PAN). The barrier polymers generally classified as moderate to intermediate include aromatic nylon MXD-6 (Mitsubishi Gas Chemical) and amorphous nylon Selar PA (Du Pont). Among the high oxygen barrier resins, the use of poly(ethylene vinyl alcohol) (EVOH) copolymers shows the most rapid growth. EVOH is commercially available in several grades with different ratios of ethylene/vinyl alcohol in the polymer chain (Eval, Selar-OH, Sarnol). Familiar containers comprising an EVOH oxygen barrier include squeezable bottles (e.g. for ketchup or other condiments), shelf-stable entree container, and, more recently, beer bottles. However, the oxygen barrier properties of high barrier polymers, such as EVOH, are still frequently not as high as glasses or metals for a wide range of packaging applications. This performance gap between high barrier polymers and glasses or metals represents a tremendous opportunity in both rigid and flexible packaging applications; this has prompted intensive effort in the field to improve the oxygen barrier performance for EVOH barrier materials.
One approach to improve oxygen barrier performance of EVOH copolymers is to incorporate platelet-type fillers into ethylene vinyl alcohol copolymers (T. C. Bissot, “Performance of High-Barrier Resins with Platelet-Type Fillers,” in Barrier Polymers and Structures, ACS Symposium Series 423 (1990), William J. Koros, Ed.). With such a composition, the oxygen barrier performance is increased approximately three-fold. The benefit is ascribed to the increased diffusion path length at the same layer thickness (tortuous path) produced by overlapping platelets obtained from orientation during processing. However, its improved barrier performance is critically dependent on the process (e.g. orientation of the platelet fillers in the EVOH matrix), and oxygen barrier performance is still not as strong as that seen for glasses and metals.
The recent development of oxygen scavenging technology has attracted significant commercial interest in the food and beverage packaging industry. With this technology, headspace oxygen in the filled package can be quickly removed by an oxygen scavenging polymer component in the packaging structure. In such a packaging structure, the headspace oxygen is consumed by the reaction between oxygen and the oxygen scavenging polymer. The reaction is often catalyzed by a transition metal salt, such as cobalt oleate. In such a system, the packaging article is typically designed to allow an efficient diffusion of headspace oxygen into the oxygen scavenging polymer in order to effect the desired oxygen scavenging reaction (oxidation), and the oxygen barrier property against ingress oxygen often relies on additional oxygen barrier layers in the packaging structures, such as aluminum foil in a juice carton packaging structures.
From this, it will be recognized that a superior oxygen barrier polymer with performance competitive with glasses and metals is extremely important to the packaging industry. Desirably, such a superior oxygen barrier polymer system would provide an extremely high oxygen barrier, or virtually zero oxygen diffusion. It would also be desirable for the superior oxygen barrier polymer to have improved moisture resistance, improved processibility, or improved interlayer adhesion.
Ching et al., WO 99/48963, showed an acrylate polymer comprising a cyclohexene moiety is very efficient in removing headspace oxygen in a packaging article. 1-cyclohexene-3-methanol was chemically linked to an ethylene methyl acrylate copolymer by a catalyzed ester exchange reaction. However, ethylene methyl acrylate is generally not regarded as being an oxygen barrier polymer.
Beer in PET, Part I of III (Packaging Strategies, Inc., eds., West Chester, Pa.) (1999), reports the reputed testing of a blend of an oxygen scavenger and an ethylene/vinyl alcohol copolymer (EVOH) in a packaging article for packaging beer. The identity of the oxygen scavenger, and the proportions of the oxygen scavenger and EVOH in the blend, were not reported. Further, this reference also does not disclose any chemical modification of EVOH.